The chemical alteration of cellulose for the purpose of introducing functional groups of specific reactivity has until now been a great preparative and technical problem. The reasons for this are mainly the following:
Cellulose contains for each glucose structural unit of its macromolecule three free but poorly reactive hydroxyl groups, a primary one on carbon atom 6 and two secondary ones on carbon atoms 2 and 3. If their reactivity is compared with that of low-molecular alkanols, the first difference is found to be that the primary hydroxyl groups are less reactive than the secondary. Then, the secondary hydroxyl group on carbon atom 3 is thus less reactive for sterical reasons than the one on carbon atom 2.
Lastly, the rest of the oxygen atoms of cellulose are in the form of an intramolecular bridge between carbon atoms 1 and 5 or of ether bridges to additional glucose units, and do not participate in reactions.
The reactions at the hydroxyl groups of cellulose are thus generally much more difficult than those of alkanols. For example, while alkanols react spontaneously and completely with isocyanates to form the urethanes, an analogous reaction with cellulose can be performed only under extreme and therefore often harmful conditions. Aside from the poor reactivity of the hydroxyl groups, it is of great importance whether they are in crystalline or in amorphous areas of the cellulose in question. The hydroxyl groups in crystalline areas are more greatly impeded in their reactivity through the formation of hydrogen bridges than those in amorphous regions.
In order to improve the reactivity of cellulose, the hydrogen bridges must be substantially broken up and also the amorphous areas must be expanded. Attempts have been made to bring this about by swelling the cellulose, but not many swelling agents for cellulose have become known. In practice, probably only water can be used, since the other agents are too expensive or too difficult to use. This not only narrows the temperature range that can be chosen, but also makes it impossible to perform reactions with reagents sensitive to water. In attempts to alter swollen cellulose chemically through its alkali salts, its great sensitivity to oxidizing influences is a great drawback.
In any case, the reaction takes place on solid structures of cellulose, which in some cases may be loosened up by swelling, i.e., it takes place in a heterogeneous phase. The disadvantages of such a method of reaction are generally well known.
It would be far more advantageous to introduce groups more capable of forming bonds into the cellulose in the dissolved form instead of the merely swollen form. The two solvents used on a large technical scale for cellulose, namely aqueous ammoniacal solutions of copper(II) tetramine hydroxide and aqueous, alkaline solutions of alkali salts of thiocarboxylic acids, are unsuitable, not merely on account of their water content, as set forth above, but also because, in many cases, a chemical reaction takes place between these solvents and the reagents provided for the functionalization of the cellulose.